Optical designers are called upon to develop ever more accurate and powerful lens systems for such applications as photolithography equipment for the manufacture of semiconductor products. These lens systems are expected to maintain an extremely high level of performance under a multitude of environmental conditions during use, storage and shipping. In addition, they must be easily, rapidly and accurately assembled. The problem, therefore, is not only to develop the proper optical design but also to mount the lenses into a final assembly which remains true to the original design and is inexpensively accomplished.
Lenses are commonly affixed within a lens cell which serves as an annular support for the lens. Typically, most lenses are held in lens cells with such mechanical devices as clips, clamps, screw-threaded retaining ring, epoxy, rolled lips or screws. The effects of thermal expansion and contraction when such a device is subjected to environmental temperature changes inevitably cause a change in the stress on the lens, manifested either as an unsymmetrical change of the clamping load on the element, possibly resulting in a translation of the element in its mounting within the cell, or an undesirable strain in the glass of the lens resulting in distortion of the transmitted wavefront.
The lenses must, in turn, be assembled together so as to constitute a unitary optical assembly. Such an assembly must hold the lenses within tight tolerances so as to ensure proper axial and radial alignment, and must be easily and accurately assembled so as to reduce the manufacturing costs to a minimum.
Lens assemblies may also be assembled together within a lens barrel which is an integral mechanical structure holding a series of lenses. It is used to position the lenses axially and radially with respect to each other, and to provide a means of interfacing the lens assembly with the system of which it is a part. Lens elements are radially positioned by the inside diameter or ID of the barrel wall. The outside diameter or OD of the lens elements are ground to fit ID of the barrel wall. The axial position of the lens elements is accomplished by cutting lens seats during assembly. The lens elements can then be constrained on the seats by epoxy, retaining rings, etc.
Such barrel mounted lens assemblies have the disadvantages of being subject to thermally induced stresses to the barrel and/or lens, require specialized materials to minimize the effect of such stresses, are difficult to machine, particularly for multiple lenses, and are inherently inflexible after machining regarding adjustment of the relative location of the lens elements during assembly.
The present invention eliminates or ameliorates the foregoing disadvantages of the prior art by providing a lens mount within a lens cell manufactured of less exotic materials which is unaffected by large temperature excursions, when returned to the nominal operating temperature range, and the same lens cell which can be used to quickly, easily and accurately assemble an optical system which achieves the original optical design requirements.